The present invention relates to a method for sputtering a dielectric target in a vacuum chamber with a high frequency gas discharge, as well a sputter target and a magnetron sputter source with a corresponding sputter target. The dielectric layer is deposited onto a workpiece, in particular a synthetic workpiece, with the aid of the known high frequency cathode sputtering method, in particular with magnetron sputtering in a vacuum chamber. Such layers are in particular applied in the production of storage plates. Among them for example optical recording methods, where the information is impressed in the plate itself and is provided with a highly reflecting layer, with a laser beam being capable of scanning the information correspondingly. In particular in the vacuum coating of optical disks with the aid of the static sputtering method with round cathode configurations with nonconducting materials, and especially in the coating of rewritable disks, good layer thickness distributions over the disk are necessary and must also be maintained over the process time, respectively the target working life.
For coating optical storage disks metallic as well as also nonconducting dielectric layers are deposited on a substrate, the data medium, the conducting metallic layers can be coated relatively simply. For coating with dielectric layers two methods are in the foreground. The one method is the so-called reactive sputtering from a conducting target. The sputtered material is herein oxidized to a nonconducting layer in the process volume or when impinging on the substrate with the aid of the reactive gas introduced into the process volume.
The other method for sputtering nonconducting substrate material is the high frequency nebulizing sputtering. Since in this case the target is not conducting, it cannot be sputtered with a DC voltage, since through the formation of a surface charge, the entire applied voltage drops over the target and not, as desired, in the plasma volume. Consequently no current can flow through the target into the plasma volume. By applying a radio frequency AC voltage the surface charges can each be led during the positive half wave from the target, the target acts like a capacitor with impedance Z =1/iωC and causes a dielectric displacement current to flow. The frequencies required for this purpose are in the high frequency range, that is in the range of >1 MHz, and the industrial frequency in the range of 13 MHz for practical reasons is a suitable choice.
To increase the target utilization and to improve the layer uniformity on the substrate, the target is eroded over a relatively large radial area. A close relationship exists between erosion profile and layer thickness distribution on the substrate. The desired erosion profile can be generated for example through a rotating magnet system, through suitably disposed magnets and pole shoes or through correction effects, for example additionally over the target working life or temporarily variable magnetic fields. Especially in statically disposed substrates, such as the storage disks, which are stationarily disposed at a distance of a few centimeters opposite a flat target, these conditions play a special role. In such vacuum coating systems the disks are cycled in through an interlock and coated in front of the target at a defined distance, with the coating time as a rule amounting to a few seconds to a few minutes. With such a sputter target thousands of disks are coated until it must be replaced after the target has been eroded so far, or when the erosion profile is too deep and the target must be replaced. Through the erosion profile, which changes over the working life of the target, the distribution conditions on the substrate also change, and this can have negative effects on the distribution especially in magnetron sputtering sources which have especially high sputtering rates and therefore are especially preferred today.
To generate a suitable erosion profile during DC sputtering, various methods are applied. For example, through the suitable disposition of the magnets which rotate in the region of the backside of the target, the most diverse erosion profiles can be generated. With the suitable choice of magnet configuration, the profile of the erosion rate can also be kept substantially constant even over the entire target working life. But it was found that this cannot be carried out in the same manner in high frequency sputtering (RF sputtering). For this reason attempts are made for example to compensate the discrepancies over the target working life with additional control magnets or with multi-part configuration of the cathode through suitable driving. These methods have been known for a relatively long time but they have the disadvantage that they are expensive in realization and that highly complex conditions obtain. The complexity of the processes increases the probability of operating errors, especially in industrial use.